Wireless communication systems are widely spread all over the world to provide various types of communication services such as voice or data. The wireless communication system is designed for the purpose of providing reliable communication to a plurality of users irrespective of their locations and mobility. However, a wireless channel has an abnormal characteristic such as a fading phenomenon caused by a path loss, noise, and multipath, an inter-symbol interference (ISI), a Doppler effect caused by mobility of a user equipment, etc. Therefore, various techniques have been developed to overcome the abnormal characteristic of the wireless channel and to increase reliability of wireless communication.
A multiple input multiple output (MIMO) scheme is used as a technique for supporting a reliable high-speed data service. The MIMO scheme uses multiple transmit antennas and multiple receive antennas to improve data transmission/reception efficiency. Examples of the MIMO scheme include spatial multiplexing, transmit diversity, beamforming, etc.
A MIMO channel matrix is formed by multiple receive antennas and multiple transmit antennas. A rank can be obtained from the MIMO channel matrix. The rank is the number of spatial layers. The rank may also be defined as the number of spatial streams that can be simultaneously transmitted by a transmitter. The rank is also referred to as a spatial multiplexing rate. If the number of transmit antennas is Nt and the number of receive antennas is Nr, a rank R satisfies R≦min{Nt, Nr}.
Meanwhile, there is an ongoing standardization effort for an international mobile telecommunication-advanced (IMT-A) system in the international telecommunication union (ITU) as a next generation (i.e., post 3rd generation) mobile communication system. The IMT-A system aims at the support of an Internal protocol (IP)-based multimedia seamless service by using a high-speed data transfer rate of 1 gigabits per second (Gbps) in a downlink and 500 megabits per second (Mbps) in an uplink. A 3rd generation partnership project (3GPP) is considering a 3GPP long term evolution-advanced (LTE-A) system as a candidate technique for the IMT-A system. It is expected that the LTE-A system is developed to further complete an LTE system while maintaining backward compatibility with the LTE system. This is because the support of compatibility between the LTE-A system and the LTE system facilitates user convenience. In addition, the compatibility between the two systems is also advantageous from the perspective of service providers since the existing equipment can be reused.
In the LTE system, up to 4 transmit antennas are supported in downlink transmission. In the LTE-A system, it is considered to support up to 8 transmit antennas in downlink transmission. As such, a new system in which the number of transmit antennas is increased in comparison with the legacy system can be taken into consideration. The greater the number of transmit antennas, the higher the information transfer rate, the reliability, etc.
Accordingly, there is a need for an apparatus and method for effective information transmission in a wireless communication system in which the number of transmit antennas is increased.